Method for manufacturing glass substrate for magnetic recording medium

ABSTRACT

The present invention relates to a method for manufacturing a disk-shaped glass substrate for a magnetic recording medium, having a circular hole at the center thereof, the method including: a shape-forming step of performing shape forming to a glass substrate having a sheet shape; a polishing step of polishing a main surface of the glass substrate; and a cleaning step of cleaning the glass substrate, in which the polishing step includes a finish polishing step of simultaneously polishing both main surfaces of the glass substrate using a polishing slurry containing abrasives having an average particle diameter of 100 nm or less; and the glass substrate polished in the finish polishing step has a thickness deviation among glass substrates polished in the same lot of 1.5 μm or less.

FIELD OF THE INVENTION

The present invention relates to a method for polishing a glass substrate to obtain a glass substrate for a magnetic recording medium, having excellent smoothness and edge shape of a main surface thereof, and a method for manufacturing a glass substrate for a magnetic recording medium.

BACKGROUND OF THE INVENTION

With increasing high recording density of a magnetic disk in recent years, characteristics required to a glass substrate for a magnetic recording medium are becoming more severe year after year. To achieve high recording density of a magnetic disk, a magnetic head is attempted to pass up to the edge of a glass substrate in order to effectively utilize an area of a main surface of the glass substrate.

In the case that the floating height of the magnetic head is reduced, if a main surface of the magnetic disk is not smooth, there is a possibility that the magnetic head contacts the magnetic disk, thereby causing a fault. Furthermore, if flatness of an edge shape of a main surface of the magnetic disk is not high, there is a possibility that when the magnetic head is passed up to the edge of the glass substrate, floating posture of the magnetic head is disturbed, and the magnetic head contacts the magnetic disk, thereby causing a fault.

In manufacturing steps of a glass substrate for a magnetic recording medium, a main surface of the glass substrate is polished using a polishing pad and a polishing slurry in order to finish to a smooth mirror surface. Polishing of the glass substrate is conducted as follows. A polishing pad comprising a polyurethane resin or the like is attached to a surface of a platen of a polishing machine, and a glass substrate and the polishing pad are relatively moved in the state that a polishing surface of the polishing pad is pressed against a main surface of the glass substrate, while supplying a polishing slurry containing abrasives between the glass substrate and the polishing pad, thereby polishing the main surface of the glass substrate.

Smoothness and edge shape of the polished main surface of the glass substrate are determined by various polishing conditions such as the kind of a polishing pad, the kind of abrasives contained in a polishing slurry, and a thickness (a removal volume) of a glass substrate removed in the polishing.

A method, which provides the standard values to all glass substrates simultaneously polished in the certain polishing machine is proposed (Patent Document 1).

However, the polishing method described in Patent Document 1 is that micro waviness (arithmetic average waviness having a wavelength of from 1.5 to 5 mm) on a main surface of a glass substrate polished is reduced to 0.6 nm or less, and does not contain any description or suggestion to further improve small micro waviness μWa (micro waviness having a period of from 50 μm to 1,000 μm).

The method for polishing a glass substrate described in Patent Document 1 is applied to a rough polishing step of a glass substrate for a magnetic recording medium using a soft polishing pad, and is not preferably applied to a finish polishing step using a soft polishing pad. In the polishing of a glass substrate using a soft polishing pad, the edge shape (dub off) of a glass substrate for a magnetic recording medium is substantially free of deterioration by increasing a removal volume. For this reason, a removal volume to the main surface at one side of the glass substrate is regulated to 9 μm or more. In the finish polishing step of a glass substrate for a magnetic recording medium, when the removal volume to the main surface at one side of the glass substrate is 9 μm or more (the total removal volume of both main surfaces is 18 μm or more), since a soft polishing pad which does not generate scratches on the main surface of the glass substrate polished is used in the finish polishing step, edge shape (dub off) of the glass substrate may be deteriorated by increasing a removal volume. Furthermore, since a polishing rate in the finish polishing step of the glass substrate for a magnetic recording medium is low, there is a possibility that polishing time is prolonged, resulting in poor productivity.

-   Patent Document 1: JP-A 2009-279696

SUMMARY OF THE INVENTION

The present invention has an object to provide a method for polishing a glass substrate, which polishes a glass substrate for a magnetic recording medium, having excellent smoothness and edge shape of a main surface in high productivity, and a method for manufacturing a glass substrate for a magnetic recording medium, including a polishing step using the polishing method.

The present invention provides a method for manufacturing a disk-shaped glass substrate for a magnetic recording medium, having a circular hole at the center thereof, said method comprising: a shape-forming step of performing shape forming to a glass substrate having a sheet shape; a polishing step of polishing a main surface of the glass substrate; and a cleaning step of cleaning the glass substrate, wherein the polishing step comprises a finish polishing step of simultaneously polishing both main surfaces of the glass substrate using a polishing slurry containing abrasives having an average particle diameter of 100 nm or less; and the glass substrate polished in the finish polishing step has a thickness deviation among glass substrates polished in the same lot of 1.5 μm or less.

The present invention can produce a glass substrate for a magnetic recording medium, having excellent smoothness and edge shape of a main surface in high productivity by setting thickness deviation among glass substrates polished in the same lot to 1.5 μm or less in a finish polishing step which polishes both main surfaces of the glass substrate for a magnetic recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a glass substrate for a magnetic recording medium.

FIG. 2 is a cross-sectional perspective view of a glass substrate for a magnetic recording medium.

FIG. 3 is a schematic view of a double side polishing machine.

FIG. 4 is a schematic view showing a measurement position of an edge shape (dub off) of a glass substrate for a magnetic recording medium.

FIG. 5 is a graph showing the relationship between thickness deviation of a glass substrate for a magnetic recording medium polished in the same lot and a total removal volume.

FIG. 6 is a graph showing the relationship between a total removal volume of a glass substrate for a magnetic recording medium and micro waviness μWa.

FIG. 7 is a graph showing the relationship between a total removal volume of a glass substrate for a magnetic recording medium and an edge shape (dub off).

DETAILED DESCRIPTION OF THE INVENTION

The embodiment for carrying out the present invention is described below, but it should be understood that the invention is not construed as being limited thereto.

The manufacturing steps of a glass substrate for a magnetic recording medium and a magnetic disk generally include the following steps. (1) A glass sheet molded by a float process or a press molding process is processed into a dick shape, and an inner peripheral side surface and an outer peripheral side surface are subjected to chamfering thereby obtaining a glass substrate. (2) Upper and lower main surfaces of the glass substrate are subjected to lapping. (3) The side surface part and the chamfered part of the glass substrate are subjected to edge surface polishing. (4) Upper and lower main surfaces of the glass substrate are subjected to polishing. The polishing step may be only primary polishing, may conduct the primary polishing and secondary polishing, and may conduct third polishing after the second polishing. (5) The glass substrate is subjected to precise cleaning, thereby manufacturing a glass substrate for a magnetic recording medium. (6) A thin film such as a magnetic layer is formed on the glass substrate for a magnetic recording medium, thereby manufacturing a magnetic disk.

In the above manufacturing steps of the glass substrate for a magnetic recording medium and the magnetic disk, glass substrate cleaning (in-process cleaning) and etching of a glass substrate surface (in-process etching) may be conducted between the respective steps. Furthermore, when a glass substrate for a magnetic recording medium is required to have high mechanical strength, a strengthening step (for example, chemical strengthening step) of forming a strengthening layer on the surface layer of the glass substrate may be conducted before the polishing step, after the polishing step or between the polishing steps.

In the present invention, the glass substrate for a magnetic recording medium may be an amorphous glass, a crystallized glass or a strengthened glass having a strengthening layer on the surface layer of the glass substrate (for example, a chemically strengthened glass). Furthermore, the glass sheet for the glass substrate of the present invention may be prepared by a float process or a press molding process.

The present invention relates to the polishing step (4) of conducting polishing on upper and lower main surfaces of a glass substrate, and relates to finish polishing of a glass substrate for a magnetic recording medium.

A perspective view of a glass substrate 10 for a recording magnetic medium of the present invention is shown in FIG. 1, and a cross-sectional perspective view of the glass substrate 10 for a recording magnetic medium is shown in FIG. 2. In those drawings, 10 shows a glass substrate for a magnetic recording medium, 101 shows a main surface of the glass substrate for a magnetic recording medium, 102 shows an inner peripheral side surface, 103 shows an outer peripheral side surface, 104 shows an inner peripheral chamfered part, 105 shows an outer peripheral chamfered part, and 106 shows an intersection between a main surface and an outer peripheral chamfered part.

Polishing of a glass substrate using a polishing pad and a polishing slurry is conducted to finish the main surface of the glass substrate 10 for a magnetic recording medium to a smooth mirror surface. A schematic view of a double side polishing machine 20 which simultaneously polishes both main surfaces of the glass substrate 10 for a magnetic recording medium is shown in FIG. 3. In FIG. 3, 10 shows a glass substrate for a magnetic recording medium, 30 shows a polishing surface of an upper platen, 40 shows a polishing surface of a lower platen, 50 shows a carrier, 201 shows an upper platen, 202 shows a lower platen, 203 shows a sun gear, and 204 shows an internal gear.

The double side polishing machine 20 rotation-drives the sun gear 203 and the internal gear 204 at a given rotation ratio, respectively, thereby moving those so as to orbit the sun gear 203 while rotating the carrier 50, and rotation-drives the upper platen 201 and the lower platen 202 in a given rotation number, respectively, thereby polishing the main surface of the glass substrate.

Polishing of the main surface of the glass substrate 10 for a magnetic recording medium is conducted after attaching polishing pads comprising a polyurethane resin or the like to facing surfaces of the upper platen 201 and the lower platen 202 of the double side polishing machine 20 and dressing polishing surfaces of the polishing pads (the polishing surface 30 of the upper platen, and the polishing surface 40 of the lower platen) into given flatness and surface roughness by a dressing jig. The glass substrate 10 for a magnetic recording medium is sandwiched between the polishing surface 30 of the upper platen and the polishing surface 40 of the lower platen in the state that the glass substrate is held on a glass substrate holding part of the carrier 50, a polishing slurry containing abrasives is supplied to both main surfaces of the glass substrate in the state that the polishing surface 30 of the upper platen and the polishing surface 40 of the lower platen are pressed to the both main surfaces of the glass substrate, respectively, and the glass substrate and the polishing surfaces are relatively moved, thereby simultaneously polishing the both main surfaces of the glass substrate.

In the finish polishing step of the present invention, a polishing pad made of a soft urethane and a polishing slurry containing abrasives having an average particle diameter (hereinafter referred to as an “average particle size”) of 100 nm or less are used as polishing tools. When a polishing pad made of a hard urethane is used as the polishing tool in the finish polishing step, there are possibilities that scratches occur on the main surface of the glass substrate polished, and it is difficult to obtain a glass substrate for a magnetic recording medium, having the main surface with high smoothness. Furthermore, when the polishing slurry containing abrasives having an average particle size exceeding 100 nm is used as the polishing tool in the finish polishing step, there is a possibility that a glass substrate for a magnetic recording medium, having the main surface with high smoothness cannot be obtained.

The average particle size of the abrasives contained in the polishing slurry used in the finish polishing step is preferably 100 nm or less, more preferably 60 nm or less, and particularly preferably 40 nm or less. In the present specification, the average particle size of the abrasives is measured using a dynamic light scattering particle size distribution measuring equipment (for example, product name: FPAR-1000AS, manufactured by Otsuka Electronics Co., Ltd.) or is measured using an electron microscope.

The abrasives contained in the polishing slurry are not particularly limited, and can be selected from colloidal silica, cerium oxide, manganese oxide, aluminum oxide and the like. Of those abrasives, colloidal silica is preferably used for the reasons that the main surface of the glass substrate can be polished with high smoothness and the abrasives deposited to the surface of the glass substrate polished can easily be removed by cleaning.

Surface characteristics of the glass substrate for a magnetic recording medium are described below. Indexes showing the surface characteristics are an arithmetic average roughness μWa of micro waviness having a period of from 50 μm to 1,000 μm and an edge shape (dub off) represented by a value of difference between the maximum value and the minimum value (the maximum value−the minimum value) from a reference line connecting both edges of a 1.6 mm-width region in a region (1.6 width) of from 0.85 to 2.45 mm from an intersection 106 between the main surface and the outer peripheral chamfered part.

As the micro waviness μWa of a glass substrate for a magnetic recording medium, the average value of the micro waviness μWa measured in an intermediate portion of a recording and reading region of the upper and lower main surfaces is preferably 0.12 nm or less, more preferably 0.11 nm or less, further preferably 0.10 nm or less, and particularly preferably 0.09 nm or less. Furthermore, the difference in the average value of the micro waviness μWa among glass substrates for a magnetic recording medium polished in the same lot is preferably 0.05 nm or less, more preferably 0.03 nm or less, and further preferably 0.02 nm or less.

In the present specification, the μWa is an arithmetic average roughness of micro waviness having a period of from 50 μm to 1,000 μm measured with a scanning white light interferometer, and a measurement region was 1.0 mm×0.7 mm.

One example of the measurement region of an edge shape (dub off) is shown in FIG. 4. In FIG. 4, 101 shows a main surface, 105 shows a outer peripheral chamfered part, 106 shows an intersection between the main surface and the outer peripheral chamfered part, and D shows a measurement region of the edge shape (dub off). In the present specification, the measurement region D of the edge shape (dub off) is set to a region (1.6 mm width) of from 0.85 to 2.45 mm from the intersection 106 between the main surface and the outer peripheral chamfered part and measurement is conducted. However, the measurement region of the edge shape (dub off) is not limited to this embodiment.

The edge shape (dub off) of the glass substrate for a magnetic recording medium is preferred as being small. When a magnetic head has passed up to the edge of the glass substrate, floating posture of the magnetic head is not disturbed, and recording and reading to the magnetic head can be stably conducted. The edge shape (dub off) of the glass substrate for a magnetic recording medium is preferably 15 nm, further preferably 12 nm, and particularly preferably 10 nm. In the present specification, the edge shape (dub off) was measured with a scanning white light interferometer.

Surface characteristics of the glass substrate for a magnetic recording medium are preferably that the average value of the micro waviness μWa measured in an intermediate portion of a recording and reading region of the glass substrate for a magnetic recording medium is preferably 0.12 nm or less, and the edge shape (dub off) is preferably 15 nm.

In the finish polishing step of the main surface of the glass substrate 10 for a magnetic recording medium, the value of the micro waviness μWa is decreased, that is, a glass substrate for a magnetic recording medium, having excellent smoothness can be obtained, with increasing a thickness (removal volume) of a glass substrate to be removed in the polishing. However, a problem may arise that the value of the edge shape (dub off) is increased with increasing a thickness (removal volume) of a glass substrate to be removed in the polishing.

To obtain a glass substrate for a magnetic recording medium, having excellent micro waviness μWa and edge shape (dub off), it is preferred that the total removal volume of both main surfaces of the glass substrate in the finish polishing step is from 0.4 to 3.0 μm. When the total removal volume of both main surfaces of the glass substrate is less than 0.4 μm, it may be difficult to obtain a glass substrate for a magnetic recording medium, having an average value of the micro waviness μWa of 0.12 nm or less. When the total removal volume of both main surfaces of the glass substrate exceeds 3.0 μm, it may be difficult to obtain a glass substrate for a magnetic recording medium, having a value of the edge shape (dub off) of 15 nm.

The total removal volume of both main surfaces of the glass substrate in the finish polishing step is preferably from 0.4 to 3.0 μm, further preferably from 0.4 to 2.5 μm, and particularly preferably from 0.4 to 2.0 μm.

In the finish polishing step of the glass substrate 10 for a magnetic recording medium, when a thickness deviation among glass substrates polished in the same lot is large, the thickness (removal volume) of the glass substrate removed in the polishing shows large variation among the glass substrates in the lot. As a result, the surface characteristics of the glass substrate for a magnetic recording medium, such as micro waviness μWa and edge shape (dub off) also show large variation among the glass substrates in the lot.

When a thickness deviation among the glass substrates polished in the same lot is large, the value of the micro waviness μWa is decreased by increasing a removal volume of the glass substrate, and the variation among the glass substrates in the lot is decreased. However, when the removal volume of a glass substrate is increased, the edge shape (dub off) may be deteriorated. Furthermore, when a polishing time is set to be long to increase a removal volume of the main surface of a glass substrate, productivity of the manufacturing step of a glass substrate for a magnetic recording medium may be deteriorated.

The thickness deviation among glass substrates polished in the same lot is 1.5 μm or less. When the thickness deviation among glass substrates polished in the same lot exceeds 1.5 μm, thickness variation in the same lot (removal volume) of the glass substrate removed in the polishing becomes large, and variation of the surface characteristics in the lot, such as micro waviness μWa and edge shape (dub off), may become large. The thickness deviation among the glass substrates polished in the same lot is 1.5 μm or less, preferably 1.0 μm or less, and particularly preferably 0.5 μm or less.

EXAMPLES

The present invention is further described below by reference to the following Examples and Comparative Examples, but it should be understood that the invention is not construed as being limited thereto.

Preparation of Glass Substrate for Magnetic Recording Medium

A glass substrate comprising SiO₂ as a main component molded by a float process was processed into a doughnut-shaped circular glass substrate (disk-shaped glass substrate having a circular hole at the center thereof) for the purpose of obtaining a glass substrate for a magnetic recording medium having an outer diameter of 65 mm, an inner diameter of 20 mm and a thickness of 0.635 mm.

The inner peripheral side surface and the outer peripheral side surface of the doughnut-shaped circular glass substrate were subjected to chamfering so as to form a glass substrate for a magnetic recording medium having a chamfering width of 0.15 mm and a chamfering angle of 45°. The upper and lower main surfaces of the glass substrate were subjected to lapping with alumina abrasives, and the abrasives were removed by cleaning.

The inner peripheral side surface and the inner peripheral chamfered part were polished with a polishing brush and cerium oxide abrasives to remove scratches on the inner peripheral side surface and the inner peripheral chamfered part, and the inner peripheral edge surface was polished so as to obtain mirror surface. The glass substrate after polishing the inner peripheral edge surface was subjected to scrub cleaning with an alkaline detergent and ultrasonic cleaning in the state of dipping the glass substrate in the alkaline detergent, thereby removing the abrasives.

The outer peripheral side surface and the outer peripheral chamfered part of the glass substrate after polishing the inner peripheral edge surface were polished with a polishing brush and cerium oxide abrasives to remove scratches on the outer peripheral side surface and the outer peripheral chamfered part, and the outer peripheral edge surface was polished so as to obtain mirror surface. The glass substrate after polishing the outer peripheral edge surface was subjected to scrub cleaning with an alkaline detergent and ultrasonic cleaning in the state of dipping the glass substrate in the alkaline detergent, thereby removing the abrasives.

Primary to Tertiary Polishing of Glass Substrate for Magnetic Recording Medium

Upper and lower main surfaces of the glass substrate after the edge processing were subjected to primary polishing with a double side polishing machine using a hard urethane polishing pad as a polishing tool and a polishing slurry containing cerium oxide abrasives (polishing slurry composition comprising cerium oxide having an average particle size of about 1.1 μm) as a main component. Cerium oxide was then removed by cleaning.

The upper and lower main surfaces of the glass substrate after the primary polishing were polished with a double side polishing machine using a soft urethane polishing pad as a polishing tool and a polishing slurry containing cerium oxide abrasives having an average particle size smaller than that of the cerium oxide abrasives used in the primary polishing (polishing slurry composition comprising cerium oxide having an average particle size of about 0.5 μm as a main component), and the cerium oxide was removed by cleaning.

The glass substrate after the first polishing and the secondary polishing was subjected to finish polishing (tertiary polishing). The upper and lower main surfaces of the glass substrate were polished with a double side polishing machine using a soft urethane polishing pad as a polishing tool of the finish polishing (tertiary polishing) and a polishing slurry containing colloidal silica (polishing slurry composition comprising colloidal silica having an average particle size of primary particles of from 20 to 30 nm as a main component).

The glass substrate after the tertiary polishing was successively subjected to scrub cleaning with an alkaline detergent, ultrasonic cleaning in the state that the glass substrate was dipped in the alkaline detergent solution, and ultrasonic cleaning in the state that the glass substrate was dipped in pure water. The glass substrate was then dried with vapor of isopropyl alcohol. Thickness (removal volume) removed in the polishing, micro waviness μWa of the main surface and edge shape (dub off) of the cleaned and dried glass substrate for a magnetic recording medium were measured.

Measurement of the thickness (removal volume) removed in the polishing was conducted by measuring the amount of change in mass before and after the polishing with a precision electronic balance (Model: HR-202i, manufactured by A&D Company, Limited) (mass method). The thickness (total removal volume) removed in the polishing was calculated by dividing the amount of change in mass before and after the polishing by specific gravity of a glass substrate and an area of the main surface. When it is desired to calculate a removal volume of the main surface of one side of a glass substrate, the amount of change in mass before and after the polishing is divided by the specific gravity of the glass substrate and the area of both main surfaces (area of the main surface×2).

The micro waviness μWa and the edge shape (dub off) were measured with a scanning white light interferometer (product name: Zygo New View 5032, manufactured by Zygo). The measurement region of the micro waviness μWa was within a region of 0.7 mm long and 1.0 mm wide in an intermediate portion of a recording and reading region of a glass substrate, and the measurement was made at positions of two places on both surfaces. The measurement region of the edge shape (dub off) was a region (1.6 mm width) of from 0.86 to 2.45 mm from the intersection 106 between the main surface of the glass substrate and the outer peripheral chamfered part as shown in FIG. 4, and the measurement was made at positions of two places on both surfaces.

The results of examining the relationship (FIG. 5) between the thickness deviation in the lot and the total removal volume of the glass substrate, the relationship (FIG. 6) between the total removal volume of the glass substrate and the micro waviness μWa, and the relationship (FIG. 7) between the total removal volume of the glass substrate and the edge shape (dub off), using a 9B double side polishing machine (product name: DSM-9B-5PV-4 MH, manufactured by SpeedFam Co., Ltd.) are described below.

Polishing of a glass substrate by the 9B double side polishing machine was carried out by setting that the number of glass substrates in one lot is 25 sheets, a main polishing pressure is 120 g/cm², a rotation number of a platen is 40 rpm, and a polishing time achieves a desired removal volume.

The result of examining the relationship between the thickness deviation in the lot and the total removal volume of a glass substrate is shown in FIG. 5. Lot having a thickness deviation among glass substrates before polishing of 3.6 μm was prepared, the polishing time was set to 20 minutes, and both main surfaces of the glass substrate were polished. Regarding 23 glass substrates in the lot, the thickness before polishing and the removal volume after polishing were measured, respectively. The thickness deviation (difference from a thickness of the thinnest glass substrate in the lot) among glass substrates polished in the same lot was plotted in a horizontal axis, and the thickness (total removal volume of the main surface) of the glass substrate removed in the polishing was plotted in a vertical axis. The removal volume of the glass substrate having a large thickness (thickness deviation value is large) in the same lot is large, and the removal volume of the glass substrate having a small thickness (thickness deviation value is small) is small. It is seen that when the thickness deviation in the lot is increased, variation of the removal volume among glass substrates polished in the same lot become large.

Lot having a thickness deviation among glass substrates before polishing of 1.0 μm or less was prepared, the glass substrate was polished by setting a polishing time to 1 minute, 2 minutes, 5 minutes, 10 minutes and 20 minutes, and the relationship between the total removal volume of glass substrates and the micro waviness μWa and the relationship between the total removal volume of the glass substrates and the edge shape (dub off) were examined. The glass substrate polished was measured on the total removal volume of the main surface, the micro waviness μWa and the edge shape (dub off). The measurement was made using 5 glass substrates every lot.

A graph obtained by plotting the total removal volume of the main surface and the micro waviness μWa is shown in FIG. 6, and a graph obtained by plotting the total removal volume of the main surface and the edge shape (dub off) is shown in FIG. 7. When the removal volume is increased, the micro waviness μWa becomes small, which is preferred. On the other hand, when the removal volume is increased, the edge shape (dub off) becomes large, resulting in deterioration.

It is seen from the result of the finish finishing of a glass substrate using the 9B double side polishing machine that when the lot having the thickness deviation among glass substrates before polishing of 1.0 μm or less is polished, the thickness (total removal volume) of the glass substrate removed in the polishing is preferably from 0.4 μm to 3.0 μm in order to obtain a glass substrate for a magnetic recording medium, having the average value of the micro waviness μWa of 0.12 nm or less and the edge shape (dub off) of 15 nm.

A 22B double side polishing machine (product name: DSM 22B-6PV-4 MH, manufactured by SeedFam Co., Ltd.) was used, the number of glass substrates polished in the same lot was increased to 180 sheets, and the relationship between the thickness deviation in the same lot and the micro waviness μWa was examined.

Polishing of the glass substrate by the 22B double side polishing machine was carried out by setting that the number of glass substrates in one lot is 180 sheets, the main polishing pressure is 80 g/cm², the rotation number of a platen is 40 rpm, and the polishing time is 20 minutes. The thickness (thickness deviation in the lot) of the glass substrate before polishing and the micro waviness μWa of the glass substrate after polishing were measured using five glass substrates in the lot.

The thickness of the glass substrate before polishing was measured with a micrometer (product name: MDC-25MJ, manufactured by Mitutoyo Corporation). The thickness of the glass substrate was measured at four points of 0°, 90°, 180° and 270° in a position of 20 mm (intermediate region of a recording and reading region) from the central portion of the glass substrate for a magnetic recording medium, and its average value was obtained. The thickness deviation among the glass substrates polished in the same lot was obtained from the difference between the maximum value and the minimum value of the thickness average values.

Lots having the thickness deviation among glass substrates before polishing of 0.5 μm (Example 1), 1.0 μm (Example 2), 2.8 μm (Example 3), 3.0 μm (Example 4), 3.3 μm (Example 5) and 4.0 μm (Example 6) were prepared, and glass substrates of each lot were polished. The results of measuring the micro waviness μWa are shown in Table 1. In Table 1, Examples 1 and 2 are Working Examples, and Examples 3 to 6 are Comparative Examples.

The lots having the thickness deviation among glass substrates before polishing of 0.5 μm (Example 1) and 1.0 μm (Example 2) were that the micro waviness μWa of the glass substrate polished is 0.11 nm or less, and the difference in the average value of the micro waviness μWa among glass substrates is 0.02 nm or less.

TABLE 1 Glass substrate, thickness (μm) Glass Thickness Example substrate Measurement Measurement Measurement Measurement Average deviation No. No. position 1 position 2 position 3 position 4 value in lot Ex. 1 1 629 629 629 629 629.0 0.5 2 629 629 630 629 629.3 3 630 629 629 630 629.5 4 629 629 629 629 629.0 5 629 629 629 630 629.3 Ex. 2 1 630 630 631 630 630.3 1.0 2 631 632 630 631 631.0 3 631 632 631 631 631.3 4 631 631 631 631 631.0 5 630 631 630 631 630.5 Ex. 3 1 629 628 629 629 628.8 2.8 2 627 628 627 627 627.3 3 626 626 626 626 626.0 4 627 627 626 626 626.5 5 628 629 628 629 628.5 Micro waviness μWa (nm) Glass Difference Example substrate Measurement Measurement Average of μWa in No. No. surface A surface B value lot Ex. 1 1 0.088 0.101 0.095 0.009 2 0.094 0.101 0.098 3 0.096 0.100 0.098 4 0.088 0.092 0.090 5 0.093 0.105 0.099 Ex. 2 1 0.094 0.091 0.093 0.021 2 0.096 0.090 0.093 3 0.114 0.113 0.114 4 0.110 0.109 0.110 5 0.097 0.107 0.102 Ex. 3 1 0.087 0.102 0.095 0.161 2 0.129 0.123 0.126 3 0.242 0.268 0.255 4 0.145 0.109 0.127 5 0.097 0.114 0.106 Glass substrate, thickness (μm) Glass Thickness Example substrate Measurement Measurement Measurement Measurement Average deviation No. No. position 1 position 2 position 3 position 4 value in lot Ex. 4 1 631 632 632 631 631.5 3.0 2 629 629 628 628 628.5 3 629 629 628 628 628.5 4 631 631 631 631 631.0 5 630 629 628 629 629.0 Ex. 5 1 629 628 628 628 628.3 3.3 2 630 631 630 630 630.3 3 628 627 628 628 627.8 4 627 627 627 627 627.0 5 630 630 630 630 630.0 Ex. 6 1 629 629 628 629 628.8 4.0 2 629 627 629 629 628.5 3 632 632 633 633 632.5 4 632 631 632 631 631.5 5 631 632 632 631 631.5 Micro waviness μWa (nm) Glass Difference Example substrate Measurement Measurement Average of μWa in No. No. surface A surface B value lot Ex. 4 1 0.110 0.116 0.113 0.125 2 0.175 0.221 0.196 3 0.172 0.204 0.188 4 0.115 0.104 0.110 5 0.211 0.258 0.235 Ex. 5 1 0.160 0.155 0.158 0.164 2 0.124 0.145 0.135 3 0.177 0.212 0.195 4 0.258 0.338 0.298 5 0.134 0.151 0.143 Ex. 6 1 0.167 0.241 0.204 0.127 2 0.162 0.291 0.227 3 0.102 0.097 0.100 4 0.102 0.113 0.108 5 0.138 0.117 0.128

The present invention can be applied to a method for manufacturing a glass substrate, including a step of polishing a glass substrate having a sheet shape. The glass substrate having a sheet shape specifically includes glass substrates for a magnetic recording medium, for a photomask, and for a display such as liquid crystal or organic EL.

While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Incidentally, the present application is based on Japanese Patent Applications No. 2010-042056 filed on Feb. 26, 2010, and the contents are incorporated herein by reference.

Also, all the references cited herein are incorporated as a whole.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   10: Glass substrate for magnetic recording medium     -   101: Main surface of glass substrate for magnetic recording         medium     -   102: Inner peripheral side surface     -   103: Outer peripheral side surface     -   104: Inner peripheral chamfered part     -   105: Outer peripheral chamfered part     -   106: Intersection between main surface and outer peripheral         chamfered part     -   20: Double side polishing machine     -   30: Polishing surface of upper platen     -   40: Polishing surface of lower platen     -   50: Carrier     -   201: Upper platen     -   202: Lower platen     -   203: Sun gear     -   204: Internal gear     -   D: Edge shape (dub off) measurement region 

1. A method for manufacturing a disk-shaped glass substrate for a magnetic recording medium, having a circular hole at the center thereof, said method comprising: a shape-forming step of performing shape forming to a glass substrate having a sheet shape; a polishing step of polishing a main surface of the glass substrate; and a cleaning step of cleaning the glass substrate, wherein the polishing step comprises a finish polishing step of simultaneously polishing both main surfaces of the glass substrate using a polishing slurry containing abrasives having an average particle diameter of 100 nm or less; and the glass substrate polished in the finish polishing step has a thickness deviation among glass substrates polished in the same lot of 1.5 μm or less.
 2. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 1, wherein a total removal volume of the both main surfaces of the glass substrate polished in the finish polishing step is from 0.4 to 3.0 μm.
 3. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 1, wherein the abrasives are colloidal silica.
 4. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 2, wherein the abrasives are colloidal silica.
 5. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 1, wherein an average value of micro waviness μWa having a period of from 50 μm to 1,000 μm measured using a scanning white light interferometer at an intermediate part in a recording and reading region on the both main surfaces of the glass substrate for a magnetic recording medium is 0.12 nm or less, and difference in the average value of the micro waviness μWa among glass substrates polished in the same lot is 0.05 nm or less.
 6. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 2, wherein an average value of micro waviness μWa having a period of from 50 μm to 1,000 μm measured using a scanning white light interferometer at an intermediate part in a recording and reading region on the both main surfaces of the glass substrate for a magnetic recording medium is 0.12 nm or less, and difference in the average value of the micro waviness μWa among glass substrates polished in the same lot is 0.05 nm or less.
 7. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 3, wherein an average value of micro waviness μWa having a period of from 50 μm to 1,000 μm measured using a scanning white light interferometer at an intermediate part in a recording and reading region on the both main surfaces of the glass substrate for a magnetic recording medium is 0.12 nm or less, and difference in the average value of the micro waviness μWa among glass substrates polished in the same lot is 0.05 nm or less.
 8. The method for manufacturing a glass substrate for a magnetic recording medium according to claim 4, wherein an average value of micro waviness μWa having a period of from 50 μm to 1,000 μm measured using a scanning white light interferometer at an intermediate part in a recording and reading region on the both main surfaces of the glass substrate for a magnetic recording medium is 0.12 nm or less, and difference in the average value of the micro waviness μWa among glass substrates polished in the same lot is 0.05 nm or less. 